Aromatic polyols are used as cross-linkers for isocyanates and isocyanurates that go into polyurethane and polyisocyanurate-based polymers. The largest end use for aromatic polyols is in applications where insulation, flammability, and structural performance are most important.
There is an increasing demand for better performing rigid polyurethane foams that have particular flammability specifications and acceptable physical properties. It is known that when typical rigid polyurethane foams, particularly spray foams, are formed in thicknesses of greater than about 2 inches, such foams are subject to internal scorching due to high exotherm temperatures resulting from reactions of certain isocyanates and polyols. Internal scorching not only degrades the physical properties of the rigid polyurethane foams rendering them unsuitable for most applications but also has the potential to cause other problems related to flammability. In addition, these typical rigid polyurethane foams are flammable and vulnerable to burning and smoking, all of which are undesirable.
To reduce scorch, decrease flammability and smoking, many rigid polyurethane foams include high levels of halogenated flame retardants. Although halogenated flame retardants are inexpensive, they have been linked to environmental concerns. Accordingly, there remains an opportunity to develop rigid polyurethane foam that has a minimum amount of halogenated flame retardants or eliminate the need to have an additional flame retardant that resists scorching, burning, and smoking, while simultaneously having acceptable physical properties.
Novolacs are known to the polyurethane industry as aromatic polyols that typically go into rigid polyurethane and polyisocyanurate foam applications. The novolac polyol is said to promote intumescence (i.e., swelling) of the rigid polyurethane foam, promotes char, decreases scorch, and decreases flammability of the foam. The novolac polyol is also thought to react with isocyanates more quickly than the isocyanates react with water thereby increasing production speed, reducing cost, and allowing the rigid polyurethane foam prepared from a novolac polyol to be used in a wide variety of applications, especially those that require fast foaming times.
While novolacs improve the flame retardancy of the polyurethane formulations and offer rigidity to the foam, these materials have drawbacks. The main challenges are the processing difficulty due to high viscosity.
Once the polyol is mixed with the isocyanates, the gel time is typically 10-25 seconds so the novolac has to mix into the system quickly, which can be a challenge due to the inherent viscosity. In addition, the urethane bond formed by the reaction of aromatic polyol and isocyanate is reversible at certain temperatures where the aliphatic polyol replaces the aromatic polyol. These factors can lead to decreased performance and difficulties in processing. While aromatic polyols offer end use benefits in polyurethanes such as flame resistance and scorch resistance they are difficult to employ in existing processes due to high viscosity and stability of the final product.
The current polyurethane formulations for applications such as rigid foam require multifunctional polyols as isocyanate reactive chemicals. The common ones are carbohydrate-based polyols, which are not very effective when it comes to flame resistance.
Thus, there is a need for aromatic polyols having decreased viscosity that will have minimal tendency to unzip in the presence of other polyols, that will increase cure efficiency, resulting in a foam with improved flammability, insulation, and mechanical characteristics compared to foams prepared with conventional polyols.